This invention relates to fire retardant thermoplastic polyesters, more particularly to poly(ethylene terephthalate) and poly(butylene terephthalate), which are fire retarded by organic phosphates or phosphonates in combination with a high charring polymer.
Fire retardant performance of polyesters can be achieved by the incorporation of various types of additives. Typically, halogenated compounds, more specifically aromatic polybrominated compounds, have been used as fire retardant additives in such polymers. It is generally accepted that these products inhibit radical gas phase reactions occurring in the flame when these products are ignited. This makes halogenated fire retardants very commonly used additives for different types of polymeric materials including polyesters. However, during the last ten years or so, halogenated flame retardants have come under scrutiny because of ecological concern, especially in Europe. At this time, the fire retardant industry is under pressure to switch to fire retardant additives that are perceived to be more environmentally friendly.
Phosphorus containing products are logical substitutes for such halogenated flame retardants. In some applications, phosphorus containing additives show as high an activity as the halogenated ones, but phosphorus containing additives are less commonly employed. They provide fire retardant activity through a combination of condensed phase reactions, polymer carbonization promotion, and char formation. These processes obviously depend on the polymer in which such additive(s) reside. Therefore, specific phosphorus containing structures need to be designed for various polymers types.
There is significant description in the prior art relating to methods of incorporating phosphorus containing additives into thermoplastic polyesters. Examples that are relevant to the present invention, but which are non-suggestive of it, are described in: U.S. Pat. Nos. 4,070,336; 4,073,767; 4,203,888; 4,257,931; 4,268,459; 4,278,591; 4,388,431; and 4,910,240; in European Patent Publication No. 791,634, and in PCT International Patent Publication No. WO 97/31 980. However, such phosphorus containing additives have limited solubility in polyesters and therefore they tend to migrate to the polymer surface. This migration phenomenon affects adversely the surface appearance and the combustion performance of the polymer product. Numerous attempts to solve this problem by copolymerization of phosphorus containing units into the polyesters chains (see, for example, E. D. Weil, Phosphorus-Containing Polymers, in the Kirk-Othmer Encyclopedia of Polymer Science and Engineeringxe2x80x9d, Vol. 11, John Wiley, New York, 1990, pp. 96-126) were not commercially successful because of their high cost and a decrease in the physical properties of the resulting polymer.
Another approach reported in the prior art deals with the preparation of polymeric additives (see, for example, U.S. Pat. Nos. 4,073,829 and 4,970,249) or the grafting of phosphorus containing groups onto a commercially available polymer different from the polyester, e.g., a phenyl-formaldehyde resin (U.S. Pat. Nos. 3,697,459, 4,010,144, or 4,105,825; or German Patent Publication No. 4,005,376; or Japanese Patent Publication No.60/137,646; or as described in R. Antony et al., J. Appl. Polym. Sci., 49 (1993) 2129-2135; R. Antony et al., J. Appl. Polym. Sci., 54 (1994) 429-438; H. Mandai et al., J. Polym. Sci., Polym. Chem., 36 (1998) 1911-1918).
More recently, Japanese Patent Publication No. 10-195283 described flame retardant polyester compositions that contained a particular phosphoric ester flame retardant, a novolak phenolic resin, and a metal oxide additive. In the Comparative Examples that were provided in this patent document, especially Examples 1-2, 5-6 and 8, it was demonstrated that polyester resin compositions containing only the phosphoric ester flame retardant and the phenolic resin did not achieve a V0 rating. The phenolic resin used in these Examples has a softening point of only about 110xc2x0 C.
The present invention relates to a polyester composition that shows high fire resistance because of the incorporation therein of a phosphorus containing fire retardant additive and a high charring polymer that has a softening point of no less than about 120xc2x0 C. The phosphorus containing fire retardant additive does not migrate to the surface of the polymer to any appreciable degree. The high charring polymer makes it possible to dissolve the phosphorus containing additives and also provides synergistic fire retardant effect with the phosphorus containing additive.
The term xe2x80x9cthermoplastic polyesterxe2x80x9d, as used herein, is intended to include any polymeric thermoplastic material composed (apart from carbons) of only ester xe2x80x94Oxe2x80x94C(O)xe2x80x94 groups in the main chain. More particularly, this invention is related, in its preferred embodiment, to the two most commonly used thermoplastic polyesters: poly(butylene terephthalate) and poly(ethylene terephthalate).
The phosphorus containing additives which are used to fire-retard thermoplastic polyesters are either diphosphates or oligomeric phosphates of the general formula: 
where R is a linear, branched or cyclic C2 to C15 alkylene group, phenylene, or bisphenylene with an alkylene, ether or sulfone bridge, Ar is an unsubstituted or substituted phenyl group, and where n can range from about 1 to about 10. This type of product is described in: U.S. Pat. Nos. 4,203,888, 4,212,832, 5,457,221, and 5,750,756, PCT International patent Publication No. WO 97/31 980 and European Patent Publication No. 791,634.
Another type of phosphorus containing additives applicable for use in the present invention are diphosphonates of the general formula: 
where R1 is linear, branched or cyclic C2 to C15 alkylene or dialkylene ether and Ar is defined as above. This type of products is described in U.S. Pat. No. 4,910,240.
The term xe2x80x9chigh charring polymerxe2x80x9d is intended to include those polymers that contain benzene rings in the main polymer backbone, that tend to produce a significant amount of char upon thermal decomposition, and that have a softening point of no less than about 120xc2x0 C. Typical commercial examples of such polymers are phenol-formaldehyde resins (including different types of substituted phenols and diphenols), polycarbonates, polyphenylenethers, polyimides, polysulfones, polyethersulfones, polyphenylene sulfides, polyetheretherketones ets. Thermoplastic polyesters (such as PBT, PET), although containing benzene ring in the main chain, do not belong to the high charring class of polymers because they tend to depolymerize and volatilize instead of charring upon heating.
The phosphorus containing fire retardant should be present at a level of at least 5% by weight of the substrate polymer and the phenol-formaldehyde resin should be present at a level of at least 5% by weight. Preferred levels for both are from about 5% to about 20%, and from about 5% to about 30%, respectively. The composition can contain one or more other functional additives that are normally employed, including: anti-drip agents; dispersants; toughening agents; and processing aid additives. Charring catalysts, including the type of metal oxide described in Japanese Patent Publication No. 10-195283, may be further added to the present compositions although the instant compositions perform in an acceptable manner in the substantial absence of such a metal oxide charring catalyst.